"I was terrified," recalls Thompson of her first lesson. "I was climbing down the ladder and clinging to the wall."

But these two middle-aged women and several of their swimming-challenged friends known as the Mermaids will join hundreds of others next Sunday jumping in the open waters off Gibson Island or the Meadowbrook Aquatic Center pool to swim a mile or three for cancer research. Organizers hope to collect $300,000 for the Sidney Kimmel Comprehensive Cancer Center at Johns Hopkins.

Phelps, the 14-time gold medalist, will be the starter for the open-water swim, which has attracted eight other Olympians and two Nobel Prize winners.

The inaugural event is part of Swim Across America, an organization that began in 1987 and has raised more than $30 million to find a cure for cancer.

The Mermaids all learned or honed their strokes with the help of the Michael Phelps Swim School and took inspiration from their coach, Annie Lawler, who was in the midst of radiation treatments for breast cancer when she agreed to be their mentor.

"We couldn't have done this without her. Annie is our hero," says Applegarth, the team captain and a cancer survivor.

It was Applegarth, a manager at the Baltimore Office of Promotion and Arts, who went from a Swim Across America organizer to in-water talent. She reached out to 10 non-swimming friends with promises of professional instruction, a swim cap and a Swim Across America magnet.

"It's all about the magnet," says Laura Lee Everett, at 43 the youngest Mermaid, as the others break into guffaws.

They can laugh about it now, but they had a rocky start.

"Nobody even knew how to put on a bathing cap," says Heidi Minken. "Really, it's not as easy as it looks."

Lawler held her breath as her charges learned to hold and release theirs. "I had no idea what was going to happen," she admits now. "I told them, 'I can meet you once a week, but you have to do the work. You need to swim three more times a week.'"

The intense schedule forced three Mermaids to drop out of the swimming program early, but the team recruited one replacement. The women, all involved in the arts, carved hours out of their workweek for practice. After four weeks, they could paddle a quarter-mile. They doubled the distance at the program's halfway point. With a week to go, all of them have mastered a mile.

Thompson now swims alone in water 8 feet deep and has learned to control her fears.

"I'm inspired by everybody I teach, but I get satisfaction every time one of them jumps into the pool," says Lawler, a Meadowbrook teacher for 15 years.

The Mermaids are proud of their aquatic accomplishments but are quick to point out that when it comes to challenges, learning to swim is nothing compared to beating cancer.

"When I was sick six years ago, I had so many people around me, at my side, in my fight. Now it's my turn to help," Applegarth says.

Everett, director of the University of Maryland Studio Opera Program, agrees. "I have a lineage of cancer in my family. It extends to friends and co-workers. I need to do this."

Their commitment and the outpouring of community and Olympic support hasn't gone unnoticed by the staff and patients at Kimmel.

"Every time I hear about the participation and who is coming, it sends chills up my spine," says Dr. Luis Diaz, an oncologist and researcher. "It does so much for what we're doing and it sends a message to our patients that they are not alone in their fight against cancer."

The money raised will be used three ways, Diaz says: to support entry-level research; to help pay for drug clinical trials; and to assist in the care and management of patients.